Winter 2009

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Lakebed Sediment Samples Reveal Size of Ancient Idaho Sockeye Runs

While the public and biologists last summer had the rare opportunity to view hundreds of sockeye salmon migrating toward Redfish Lake near Stanley, an Idaho State University researcher had his sights on ancient salmon runs that were hundreds of times larger.

The runs that Bruce Finney, Ph.D., was focused on happened centuries ago. While tourists stood streamside and watched the fish migrate in the upper Salmon River, Finney had to use a sophisticated scientific instrument to measure the vastly greater numbers that journeyed upstream from the Pacific Ocean as long as 2,000 years ago.

Fishery managers were thrilled by the return of more than 550 sockeye salmon to the Sawtooth Valley last summer. The fish had migrated upstream nearly 900 miles to reach the Sawtooth Fish Hatchery and Redfish Lake Trap.

In the past, however, the run consisted of tens of thousands of fish, Finney says.

In some recent years, there have been only single-digit or zero returns of the sockeye. While this year's relatively strong run was encouraging to those who want to save the run, its size was a small fraction of the runs that previously returned to spawn in the lakes of the upper Sawtooth Valley, including Redfish, Petit and Alturas Lakes.

Prior to the 1900s, Finney says, sockeye spawning in Redfish Lake used to number 25,000 to 40,000. He bases this calculation on sediment cores taken from lakes and examined using a mass spectrometer.

Mass spectrometers can measure the masses and relative concentrations of atoms and molecules in chemical compounds and other samples. They accurately measure the different types of isotopes of the same element, such as carbon or nitrogen in biological or geological samples.

Finney developed a technique using mass spectrometers to reconstruct the salmon runs in the past. He wrote about the technique in the prestigious journals, Science and Nature.

Salmon have a signature type of nitrogen, recorded by nitrogen isotopes, that scientists can use to positively identify as originating from the salmon.

When salmon die and decompose, they release nitrogen, which is taken in by algae that end up in lake sediments. High salmon runs correspond to higher concentrations of these nitrogen deposits; lower concentrations of nitrogen in deposits represent smaller runs.

The sediments on lake bottoms can be dated using radiocarbon dating, an accurate method for dating materials, and by identifying ash layers from known volcanic events.

So far, Finney and his colleagues have studied core samples from some of the larger lakes in the upper Sawtooth Valley, including Redfish Lake, dating back about 2,000 years.

Finney's mass spectrometer measurements confirm anecdotal and eyewitness accounts of what led to the precipitous demise in the Idaho upper Salmon River sockeye population: construction in 1910 of the Sunbeam Dam, about 15 miles downstream of Stanley.